The result could trigger another potentially violent fallout between the governing party and the opposition following the disputed election five years ago.
One of the main issues is the status of Zanzibar's union with mainland Tanzania.
Al Jazeera's Catherine Soi reports from neighbouring Kenya.
- Subscribe to our channel: http://aje.io/AJSubscribe
- Follow us on Twitter: https://twitter.com/AJEnglish
- Find us on Facebook: https://www.facebook.com/aljazeera
- Check our website: https://www.aljazeera.com/